In recent years, in view of improvement of fuel efficiency or the like, expectations of weight reduction of a vehicle such as a vehicle body or an airframe have increased. For this reason, a technique to use a three-dimensional solid knitted fabric, which is lighter than urethane foam, as a cushioning member even in a seat used in those vehicles, or the like, has also been considered. However, not only such weight reduction of a cushioning member, but also further thinning of a metal plate used as a frame can also be thought as a way to reduce the weight of a seat. That is, a frame is made by making two thin metal plates face each other and joining their end edge portions to each other, but, in a case where such a frame made of thin metal plates (a thin plate frame) is used, trying to couple the thin plate frames each having a thickness of, for example, 1.0 mm or less, with each other by welding is likely to cause material degradation or a welding defect, since the heat capacity is high. On the other hand, in view of troublesome work, time, and costs for welding thin plate frames, a technique to perform hemming process on a pan frame at an end edge is disclosed in Patent Document 1.